The invention relates to the area of mechanical design and to connections/joints between assembled mechanical components.
Many mechanical systems such as precision machine tools and instruments, robots, etc., comprise structural blocks attached to other structural blocks through surface contact connections. The connections can be permanent, such as a bolted connection between the headstock and the bed of a lathe. Another group is infrequently disconnectable systems, such as so-called xe2x80x9creconfigurable machining systemsxe2x80x9d composed of standard units assembled in various combinations for using in a production line for a certain product and reconfigured for fabrication of a new product. The third widely used type of connections is for connecting interchangeable tools, measuring heads, etc., in a precision location to permanent structural components, such as spindles of machining centers or turrets of lathes. In all these three cases, but especially in the second and third ones, high precision of the assembled systems is required, thus an adjustment of the final assembly is often desirable.
In the first case (permanent assembly) the connected parts are often fabricated for fitting the designated specific counterparts, and the connection may be finish-machined during the assembly process.
Such an expensive procedure cannot be accepted for assembly of a reconfigurable machining system. In this case, no finish machining can be tolerated during the assembly, since each unit has to be suitable for connecting with any other unit of the system, so that any xe2x80x9cfinishingxe2x80x9d would damage the whole system. In such circumstance, an adjustability built into the system design would be very desirable. Unfortunately, no adjustable connections are available, and usually flat contact surfaces preloaded by bolts are used as connections. Their dimensions can be adjusted somewhat by changing the preloading force, but reduction of the preloading force results in a significant and often unacceptable reduction of stiffness of the connection, while increase of the preloading force results in undesirable reduction of damping.
Even more interchangeability is required for connecting tools and measuring heads with the base system in the third case. Both high accuracy and overall tightness for achieving high stiffness (xe2x80x9cperfect fitxe2x80x9d to realize a simultaneous contact both on tapered surfaces and on the face surfaces of the connection) are required. However, it would be prohibitively expensive to standardize extremely tight tolerances for tens of thousands spindles and turrets and for millions of toolholders, for them to be able to perfectly fit each other in random combinations. Thus, the adjustability or means for compensating dimensional variations are needed even more.
Sometimes in all these cases a specified stiffness of the connection is required. However, conventional surface contact connections are highly nonlinear and any change in preloading force changes the stiffness.
The need for compensation ability is the most clearly understood in application to the last case (tool interchange), and is realized by designing elastic deformations into the system, especially into toolholder/spindle interface system.
There are two basic systems for incorporating flexibilities into the toolholder/spindle interface system.
One technique is represented by tapered toolholders HSK (German DIN Standard) and KM (Kennametal Corp.), both described in Rivin E.I, xe2x80x9cTooling Structure: Interface between Cutting Edge and Machine Toolxe2x80x9d, Annals of the CIRP, vol. 49/2/2000, pp. 591-634, wherein the tapered body to be fit into the reciprocating tapered hole in the spindle/turret is a high precision hollow structure slightly deforming when pulled in by the drawbar, thus realizing the xe2x80x9cperfect fitxe2x80x9d with the simultaneous taper/face contacts. Very shallow taper connections ({fraction (1/10)}) are used in these systems in order to increase the mechanical advantage and thus to facilitate the deformation of the rather rigid structures. Shortcomings of this technique are the costs of precision fabrication of a complex shape; a large variation (about 2:1 even for the standardized very high precision) of the degree of interference between the male and female tapers resulting in the reduced performance consistency; reduced effective stiffness of the clamped tools due to increased overhang caused by the hollow structure of the toolholder (e.g., see the above quoted article).
Another technique is represented by U.S. Pat. Nos. 5,322,304 (the Prior Art) and 5,595,391, both granted to the present inventor. FIGS. 1, 2, 3 from U.S. Pat. No. 5,322,304 show toolholder 60 to whose tapered surface precision balls 68 are attached by means of cage 66 as precision flexible elements. When the toolholder is inserted into tapered spindle hole 14 and pulled into it by the drawbar (not shown, is engaging with part 60b by threaded adapter 22), radial deformations of balls 68 allow for toolholder 60 to move inside spindle hole 14 as much as needed in order to achieve the simultaneous contact between the male and female tapered surfaces (via balls 68) and also between flange 60c of the toolholder and face 16 of the spindle. Since high precision balls of various diameters and materials are available off-the-shelf and are inexpensive, and since the required modification of the standard toolholders (reducing diameter of the tapered part to accommodate the balls) does not increase their design complexity and costs, this system works reasonably well. However, it is usually applied to the so-called xe2x80x9csteep taperxe2x80x9d ({fraction (7/24)} taper) standard toolholders whose multi-million inventory is widely used in manufacturing plants. These toolholders, as standardized, have rather loose tolerances and also are often used with reground spindles or turrets thus further increasing the scatter of the dimensions and, effectively, loosening the tolerances and expanding requirements to compensation of the axial distance between the spindle face and the toolholder flange. Considering these factors, the required axial dimensional compensation is up to 150-200 xcexcm requiring radial deformation up to 30 xcexcm of the flexible elements attached to the toolholder. However, the safe allowable elastic deformation of precision steel and titanium balls of typical 5 mm diameter is only about 5-10 xcexcm (0.1-0.2% relative compression).
Dynamic stability and other performance characteristics of modern high speed/high power/high accuracy machines are dependent on their structural stiffness but also on damping which is largely determined by the structural connections, e.g. see Rivin, E.I, xe2x80x9cStiffness and Damping in Mechanical Designxe2x80x9d, Marcel Dekker, 1999. The techniques mentioned above for achieving the simultaneous taper and face contact between the toolholder and the spindle flange unfortunately do not increase damping in the connection. While both stiffness and damping are to a large extent controlled by connections/joints between the mechanical components, the stiffness is increasing with increasing contact pressures in the joints but damping is changing in the opposite direction, e.g., see the above quoted book. At low contact pressures xcx9c1 MPa (150 psi), damping in a flat joint is characterized by log decrement xcex4=xcx9c0.075, but the stiffness of such joint is inadequate for many applications. Increase of the contact pressure to xcx9c3 MPa (450 psi) results in xcx9c1.5 times stiffness increase but damping falls to xcex4=0.03. In critical applications, expensive and often bulky special damping means are used, such as squeeze film dampers or dynamic vibration absorbers.
The instant invention provides means for solving the above-addressed problems and eliminating or alleviating the mentioned shortcomings of the conventional mechanical connections by inserting segments of precision cylinders, made from materials whose Young""s modulus is at least one-tenth of the lowest Young""s modulus of the materials of the connected mechanical components, between the contact surfaces of the mechanical components being connected, thus resulting in high stiffness or in high stiffness/high damping combination in mechanical connections/joints while in the same time being robust and not significantly influencing costs and weight of the systems where the proposed technique is used.
A design technique for a connection between two conforming and pressed together surfaces is disclosed, in which intermediate cylindrical segments of uniform cross sectional diameter and having initial line contact with at least one surface are inserted between the joined surfaces.
According to the invention, the connection is preloaded, thus causing radial elastic deformation of the cylindrical segments.
Depending on the design needs, the stiffness of the connection can be adjusted by using cylinders with different diameters, with round or elliptical cross sectional shapes, with different ratios of the internal and external diameters (the limiting case being the internal diameter equal zero), and different materials.
The proposed technique allows to perform a fine adjustment of the linear and/or angular positioning of the connected components without stiffness change of the connection.
According to another feature of the invention, introduction of cylindrical segments into the connection allows to compensate dimensional variation of the connected mechanical components and to resolve statically indeterminate situations.
According to a further feature of the invention, the cylindrical elements are made from a shape memory or a superelastic material, allowing to realize an extremely large range of fine adjustment, while exhibiting a very significant amount of damping.